Pipelines are used to transport fluids such as the production fluids from oil and gas wells. Because the measurement of these fluids is important, orifice plates are installed in special fittings which are installed in-line with pipeline sections. Some fittings may permit an orifice plate to be moved in and out of the flow stream without interruption of the flow through the pipeline. Other fittings permit orifice plates to be moved in and out of the pipeline only by interruption of flow.
The use of orifice measurement for flow has been known since ancient times. The basis of orifice measurement is to place a plate in a flow line, with the plate having an opening which is smaller than the opening of the flow line. By reading the upstream and downstream pressure on either side of the plate, and calculating the difference of pressure between the upstream and downstream pressures, one can infer the rate of flow in the pipe line.
The accuracy of the measurement given by the orifice is dependent on many factors, including the ratio of the orifice hole to the diameter of the pipe, the length of straight run of the upstream and downstream pipe or tube sections on either side of the orifice, the eccentricity of the orifice hole in the pipe or tube and the like. The standards to assure accuracy have been governed by ANSI Standard ANSI/API 2530, which are wholly adequate for flow measurement.
Nevertheless, there has been a series of meetings to significantly revise API/AGA standards, which would constitute a revision to Manual of Petroleum Measurement Standards, Chapter 14, "Natural Gas Fluids Measurement", which includes Section 3, "Concentric, Square-edged Orifice Meters". The revised Section 3 may become an update and would then become a revision to ANSI Standard ANSI/API 2530; i.e.: AGA Report No. 3.
These new requirements probably will be adopted by some companies even though additional costs will be attendant with mechanical changes associated with the requirements. The new requirements require greater control on centering (concentricity or eccentricity) of orifice plates as installed in any holding device. However, most orifice plate holding devices other than flanges require some manner of plate/seal/carrying device combination. The most common seal of this type is an elastomeric seal which is assembled around the orifice plate. This type of seal may not yield the greater control of centering required by the new revisions with respect to installed assemblies. This is due to molding tolerances.
There is, therefore, needed an orifice plate and seal combination that permits greater control of concentricity at installation while still permitting the current common sealing aspects presently in use.